The charge pump-type DC-DC converters allow high voltage to be obtained with high efficiency for a low current load. Further, the charge pump-type DC-DC converters dispense with components such as a transformer and an inductor, so that all circuits can be integrated into a single IC. Accordingly, the charge pump-type DC-DC converters have been employed in a wide variety of circuits such as a memory circuit, a CCD drive circuit, and an LCD drive circuit.
However, the conventional charge pump-type DC-DC converters have only been able to generate a voltage that is an integral multiple of an input voltage. In order to solve this problem, a charge pump-type DC-DC converter additionally including a variable voltage source at a previous stage so as to be able to set an input voltage to a desired value to obtain a desired output voltage has been proposed. Japanese Laid-Open Patent Application No. 5-111241 discloses such a charge pump-type DC-DC converter.
The conventional charge pump-type DC-DC converter having a variable voltage source at a previous stage employs a constant voltage circuit as the variable voltage source, and includes an overcurrent protection circuit in the constant voltage circuit.
FIG. 1 is a circuit diagram showing a conventional charge pump-type DC-DC converter 100. The DC-DC converter 100 includes a constant voltage circuit part 101 and a charge pump circuit part 102. Further, the constant voltage circuit part 101 includes a voltage control part 103 and an overcurrent protection circuit part 104. The voltage control part 103 includes a voltage control transistor Ma, resistors Ra and Rb generating and outputting a detection voltage VdA proportional to the output voltage VoA of the constant voltage circuit part 101, a reference voltage generator circuit 111 generating a predetermined reference voltage VrA, and an operational amplifier AMPa.
The operational amplifier AMPa controls the gate voltage of the voltage control transistor Ma so that the detection voltage VdA is equalized with the reference voltage VrA. As a result, the output voltage VoA of the constant voltage circuit part 101 is equal to VrA×(Ra+Rb)/Rb, where Ra and Rb indicate the resistances of the resistors Ra and Rb, respectively.
The overcurrent protection circuit part 104 includes a current detection transistor Mb detecting the current flowing through the voltage control transistor Ma, a resistor Rc converting the current flowing through the current detection transistor Mb into voltage, an operational amplifier AMPb comparing the voltage generated in the resistor Rc and the detection voltage VdA, and a current control transistor Mc controlling the output current of the voltage control transistor Ma.
Referring to FIG. 2, when the output current ioA of the constant voltage circuit part 101 increases to ia, the voltage at the inverting input terminal of the operational amplifier AMPb exceeds the detection voltage VdA to switch on the current control transistor Mc. As a result, the gate voltage of the voltage control transistor Ma increases. Accordingly, the output voltage VoA of the constant voltage circuit part 101 decreases, and an increase in the current ioA output from the voltage control transistor Ma is controlled. When the output voltage VoA of the constant voltage circuit part 101 decreases, the detection voltage VdA also decreases. Accordingly, the current ioA output from the voltage control transistor Ma starts to decrease. This relationship between the output voltage VoA and the output current ioA is shown in FIG. 2. In FIG. 2, the characteristic indicated by OUTa represents the relationship between the output voltage VoA and the output current ioA of the constant voltage circuit part 101, and the characteristic indicated by OUTb represents the relationship between the output voltage VoB and the output current ioB of the charge pump circuit part 102.
As shown in FIG. 2, when the output voltage VoA of the constant voltage circuit part 101 decreases to 0 V, the output current ioA decreases to only ib. This is because if the overcurrent protection circuit part 104 reduces the output current ioA to 0 A, the output voltage VoA may not rise in the case of rising from 0 V. When the output voltage VoA of the constant voltage circuit part 101 is 0 V, a positive offset voltage is caused to be generated at the inverting input terminal of the operational amplifier AMPb so that a current of ib flows as the output current ioA. The offset voltage may be generated by, for instance, changing the size of each of the transistors used for the two inputs of the operational amplifier AMPb.
The charge pump circuit part 102 includes switch elements SWa, SWb, SWc, and SWd, which are MOS transistors, capacitors Ca, Cb, and Cc, and a clock generator circuit 112 controlling the switching of the switch elements SWa through SWd. The clock generator circuit 112 generates and outputs clock signals CLKa, CLKb, and CLKc. The switching of the switch element SWa, the switching of the switch element SWd, and the switching of the switch elements SWb and SWc are controlled by the clock signals CLKa, CLKb, and CLKc, respectively. The clock signals CLKa and CLKb are opposite in phase. The switch element SWa is switched ON when the level of the clock signal CLKa is LOW, and the switch element SWd is switched ON when the level of the clock signal CLKb is HIGH. Accordingly, the switch elements SWa and SWd are switched ON and OFF simultaneously.
The switch elements SWb and SWc are switched ON when the level of the clock signal CLKc is LOW. When the switch elements SWa and SWd are switched ON, the capacitor Cb is charged with the output voltage VoA of the constant voltage circuit part 101. When the switch elements SWb and SWd are switched ON, the voltage of the capacitor Cb is added to the capacitor Ca, so that the capacitor Cc is charged with the combined voltage. Accordingly, the voltage of the capacitor Cc, which is the output voltage VoB of the charge pump circuit part 102, is double the output voltage VoA of the constant voltage circuit part 101. Since the output voltage VoB of the charge pump circuit part 102 is double the output voltage VoA of the constant voltage circuit part 101, the output current ioB of the charge pump circuit part 102 is half of the output current ioA of the constant voltage circuit part 101 as shown in FIG. 2.
However, according to such a conventional circuit, even when the output terminal OUTb of the charge pump circuit part 102 is short-circuited to ground, the output voltage VoA of the constant voltage circuit part 101 is prevented from becoming 0 V, and is only reduced to a voltage Vs shown in FIG. 2. This is because the switch elements SWa through SWd are provided between the output terminal OUTa of the constant voltage circuit part 101 and the output terminal OUTb of the charge pump circuit part 102. This causes the problem that a current ic, larger than the current ib that flows when the output terminal OUTa of the constant voltage circuit part 101 is short-circuited to ground, flows as the current ioA supplied from the constant voltage circuit part 101.